1. Field of the Invention
This invention relates to plant food solutions, and more particularly, to storage stable liquid compositions comprising urea, uncondensed methylolurea, and water, combined as plant food solutions containing high nitrogen concentrations which exhibit properties of low saltout temperatures and low phytotoxicity.
2. Description of the Prior Art
Urea is one of the most economical and widely used plant food nitrogen sources. It is mainly used as a granular solid or as a mixed aqueous solution with an ammonium salt such as nitrate. The mixed solutions are necessary to achieve the commercially necessary high nitrogen concentrations and low saltout temperatures. Saltout temperature (hereinafter called SOT) is that temperature at, and below, which a plant food solution is no longer clear because one, or more, of the solution constituents has precipitated. Unfortunately, aqueous urea solutions containing substantial nitrogen concentrations have SOT's which are too high for general commercial use. For example, the maximum nitrogen concentration which may be achieved in an aqueous urea solution having a SOT of 0.degree. C. is 16 weight percent.
Urea in aqueous solution frequently causes plasmolysis, or burn, in plants treated with the required amounts of nitrogen, particularly where the solution is allowed to contact the foliage of the plants. Mixed aqueous solutions containing urea with an ammonium salt, such as nitrate or sulfate, have even higher burn potential than does aqueous urea.
The burn tendency of urea has been reduced by many workers in the field by condensing it with formaldehyde to produce slow releasing urea polymers as indicated by Tisdale and Nelson on pages 174-176 in the Second Edition of Soil Fertility and Fertilizers.
Kealy in U.S. Pat. No. 3,235,370 provided a non-condensed urea-formaldehyde liquid suspension which was storable for 30 days. He did not provide an economical urea, urea-formaldehyde, water solution; a clear solution storable for about one year or more; nor an eutectic urea, methylolurea, water solution having a low SOT. U.S. Pat. No. 3,462,256, to Justice, et al, discloses a concentrated urea-formaldehyde solution containing 20 percent water or less in which between 50 and 80 percent of the formaldehyde is present as methylene ureas.
U.S. Pat. No. 4,304,588 to Moore discloses a clear storage stable, concentrated urea-formaldehyde-based solution containing 50 percent or more of the urea in the form of methylolurea with an overall molar ratio of urea to formaldehyde between 1.4 and 1.9, and a process for the preparation of this solution. Although the solution disclosed was low in the potential for burning plant foliage, no teaching was provided regarding economical eutectic solutions containing urea, methylolurea, and water, having high total nitrogen concentrations and low SOT's. The addition of urea to the product of U.S. Pat. No. 4,304,588, or to other urea-formaldehyde products of the prior art, to improve economics caused a significant increase in the SOT of the mixture, to a point where it is not useful as a plant food solution under normal circumstances.
Thus, no teaching was available from the prior art which would allow the blending of low-burn liquid urea-formaldehyde products with low cost nitrogen plant food chemicals to produce economical plant food solutions which were useful under a wide range of storage times and temperatures, while still retaining a lower tendency to cause burn to treated plants than urea or inorganic nitrogen fertilizers. Such a liquid plant food composition has been especially needed for fertilization of turf and other grasses, and for foliar feeding of many crop and ornamental plants and could be used as a row crop fertilizer or as a manufacturing solution for other liquid or solid fertilizers.